Four stunning optical phenomena and how they form

We’ve all looked up at the sky at some point and seen weather-related spectacles overhead. The chances are that they were caused by the interaction of light and matter – known as optical phenomena.

While the list of optical phenomena that can occur is more than you might imagine, there are a selected few that are the most common.

I thought I’d take a look at the ones that you’ve probably seen – rainbows, halos, crepuscular rays and circumzenithal arcs – and explain how they form.

Rainbows

Rainbows are caused by the reflection and refraction of light in water droplets that lead a the spectrum of light appearing in the sky.

They only occur when the sun is behind you because the light needs to shine into raindrops in front of you, then bounce back towards your eyes.

Rainbows are most common in the spring and summer when the sun spends more time at a lower position in the sky, although they can happen in summer early or late in the day when the sun isn’t as high overhead.

Circumzenithal arcs

Circumzenithal arcs look very much like rainbows, except that they only occur in the sky and are instead upside down.

They are caused by the refraction of sunlight through horizontally-oriented ice crystals, most often in cirrus clouds, where ice crystals are prevalent.

Circumzenithal arcs can only form when the sun is at an altitude low than 32.2 degrees and are brightest when the sun is 22 degrees above the horizon.

Halos

Halos are similar to circumzenithal arcs in the sense that they occur due to presence of ice crystals, although from cirrostratus cloud, rather than cirrus.

The ice crystals in cirrostratus clouds act as prisms and mirrors, refracting and reflecting the sunlight, pushing out shafts of light in different directions.

The end result is a circular ring around the sun that is white or faintly coloured and seems to radiate light outwards.

Crepuscular rays

Crepuscular rays are shafts of sunlight that emanate from clouds, normally stratocumulus, when the sun is behind them.

This generally occurs during the hours around dawn and dusk, when the sun is low enough in the sky to sit behind cloud cover.

As the shafts of light stretch out from the cloud, they alternate with less lit zones to give a fan-like effect.

If you ever manage to capture optical phenomena on camera, send them to me on Twitter – @liamdutton